1. Field of the Invention
This invention relates to linear motors used in super-high speed transportation systems, and more particularly to a power supply system for supplying power to driving coils of an elongated linear motor system such as a linear induction motor system, linear synchronous motor system or d.c. linear motor system disposed on the ground. The present invention has for its object to provide a highly economical and reliable power supply system capable of supplying power effectively to driving coils which constitute a load of low utility factor.
2. Description of the Prior Art
According to a method proposed hitherto for supplying power to the driving coils of an elongated linear motor system, the elongated linear motor is divided into a plurality of major sections and a power supply unit is provided for each of these sections for supplying power thereto. While this method has been advantageous in respect of the power supply equipment, it has been uneconomical in that the capacity of the power supply equipment is excessively large due to large reactive power requirements. In an effort to obviate the above disadvantage, a method has been proposed according to which each of such major linear motor sections (which are called so hereinafter although they are actually the driving coils) is suitably divided into a plurality of unit linear motors and power is successively supplied to these unit linear motors as a carriage moves therealong. The present invention is concerned with a power supply system for supplying power to such unit linear motors. According to the prior art proposal, a power supply unit consisting of, for example, a voltage step-down transformer and a converter such as an inverter, cycloconverter or chopper is connected to each unit linear motor and the power supply units are successively switched over with the movement of a carriage. While this method has been advantageous in that supply of reactive power can be eliminated and the power supply units need not have any excessive capacity, it has been uneconomical in that the power supply units are required in a large number. A method has been proposed to obviate the above disadvantage. According to this proposal, a single power supply unit is connected to the unit linear motors through mechanical switches or thyristor type electrical switches and power is supplied to these unit linear motors by successively turning on and off these switches. While this method has been economical, it has had the following defects: The number of the unit linear motors associated with a moving carriage varies with the movement of the carriage. Thus, (1) the power supply unit must have a capacity which can simultaneously feed a plurality of the unit linear motors, resulting in redundancy of the capacity in the state in which the number of the unit linear motors simultaneously supplied with power is less than the predetermined maximum. Suppose, for example, that the zone of each unit linear motor is longer than the length of the carriage and 100% power is supplied to one of the unit linear motors when the carriage exists on such unit linear motor. In this case, 200% power must be supplied when the carriage is moving across two of the unit linear motors, and the power supply unit must have a 200% capacity. (2) In the state in which a plurality of these unit linear motors are simultaneously supplied with power, they are connected in parallel to the same power supply unit. In this case, any unbalance between the individual unit linear motors results in unbalanced supply of power to these unit linear motors. Further, when the converter of the kind above described is incorporated in the power supply unit, this converter is limited to the voltage type. Thus, this power supply system cannot be universally employed in view of the fact that the converter is preferably of the current type in the case of the power supply unit for a linear synchronous motor or d.c. linear motor. (3) The switches must have a sufficient breaking capacity due to the fact that the switches are opened in the state in which current is conducted therethrough. (4) The above defect may be obviated by switching over the switches in the state in which any substantial power is not supplied from the power supply unit. However, this is objectionable for the purpose of propulsion of the carriage since any substantial power is not supplied to the linear motor during this period of time. (5) Due to the fact that only one power supply unit is provided for the linear motor section and all the unit linear motors belonging to this section are fed from this power supply unit, no power can be supplied to this linear motor section in the event of failure of the power supply unit, thereby obstructing the movement of the carriage. Thus, this power supply system is not fully reliable.